edit:
Not sure why I'm getting downvoted, maybe a link to a source will help.
"Everything in your Dropbox folder is always on your hard drive and merely synced with our online service, so you have access to your files even if you're offline. If you have multiple linked computers on your local area network, we'll even sync from those computers rather than via the Dropbox service whenever it can."
There was a bug a couple months ago where Dropbox zero'd out some files, and then synced the 0 byte files across all of your connected computers, thereby destroying every linked Dropbox file that was affected.
There was that bug a few months ago where Dropbox left all accounts open, with no passwords, for four hours or something like that.
They are not resistant to failure by design.
Dropbox is not the shining example of corporate citizenry, especially when it comes to security (although they're fairly good on uptime).